1. Field of the Invention
The subject invention pertains to the art of antennas and specifically to a cross polarization compensated feed array for a dome lens antenna.
2. Description of the Prior Art
Dome antenna systems such as that disclosed in U.S. Pat. No. 3,755,815, issued Aug. 28, 1973 to Stangel et al. and assigned to the assignee of the present invention achieve hemispherical scan coverage with a single active planar phased-array feeding a dome shaped lens. This novel antenna design offers a number of significant advantages over conventional multifaced planar arrays including: substantially reduced cost; reduced complexity; and increased scan coverage, having greater than hemispheric scan capabilities. The dome antenna, however, exhibits undesirable cross polarization characteristics created by the depolarizing effects of the conformal surface of the dome shaped lens. This problem is exacerbated by the variation of the refractive index of the dome as a function of elevation angle. Additionally, the distributive source characteristics of the feed system of the lens provides an illumination which varies with scan angle. These factors contribute to the polarization distortion realized by a dome antenna system. When the feed array is designed for monopulse operation, the cross-polarized components in the far-field radiation pattern, generated by this polarization distortion, fill the central null in the monopulse difference pattern, thereby degrading the angular tracking accuracy of the radar system. To provide monopulse radar high precision tracking, with the dome antenna system, it is necessary to substantially eliminate the null filling caused by the cross-polarization components.
One proposed solution to the problem introduces independent polarization control in each element of the feed array. For dome antenna systems, the cost of implementing such a scheme would be prohibitive, requiring two phase shifters (one for each polarization) and a preprogrammed attenuator for each element in the array, which may number over a thousand. Another proposal introduces appropriate amplitude weighting to the elements for each scan angle to reduce the cross-polarization level in the antenna difference pattern. This method is also complex and expensive.